The team of adventurers and engineers behind the Solar Impulse project are already celebrating an aviation milestone for the longest solar-powered flight after keeping the single-seat prototype aloft for almost 15 hours.

But with the goal of 24 hours of nonstop flight, the team set its sights on keeping the sleek aircraft with a 207-foot wingspan in the air until this morning.

Pilot Andre Borschberg "will stay up there as long as possible," said Bertrand Piccard, the project's co-founder.

Sign Up and Save

"Hopefully he will still be in the air at sunrise.... That is the challenge."

Borschberg took off from Payerne airfield into the clear blue sky shortly before 7 a.m. Wednesday, allowing the plane to soak up plenty of sunshine and fly in gentle loops over the Jura mountains west of the Swiss Alps.

As the sun set, technicians hoped the Solar Impulse's batteries — charged from the 12,000 solar cells fixed to the wings and body — would keep the four-engine plane airborne through the night. The batteries would begin charging again at dawn.

Earlier in the afternoon, Piccard said the flight was going "extremely well."

A record-breaking balloonist whose father and grandfather also accomplished pioneering airborne and submarine feats, Piccard has become the figurehead for the project and will be one of two pilots when it eventually takes off for its round-the-world attempt in 2013, with a scheduled five stops along the way.

Piccard said the night flight was a key step toward achieving that ultimate feat.